Fatal error: Uncaught Error: Call to undefined function set_current_screen() in /var/www/html/wp-admin/admin-header.php:30 Stack trace: #0 /var/www/html/wp-content/plugins/civicrm/civicrm/CRM/Utils/System/Base.php(267): require_once() #1 /var/www/html/wp-content/plugins/civicrm/civicrm/CRM/Utils/System.php(219): CRM_Utils_System_Base->theme('<div id="crm-co...', false, false) #2 /var/www/html/wp-content/plugins/civicrm/civicrm/CRM/Core/Error.php(233): CRM_Utils_System::theme('<div id="crm-co...') #3 /var/www/html/wp-content/plugins/civicrm/civicrm/packages/PEAR.php(921): CRM_Core_Error::handle(Object(DB_Error)) #4 /var/www/html/wp-content/plugins/civicrm/civicrm/packages/DB.php(985): PEAR_Error->__construct('DB Error: alrea...', -5, 16, Array, 'CREATE TABLE ci...') #5 /var/www/html/wp-content/plugins/civicrm/civicrm/packages/PEAR.php(575): DB_Error->__construct(-5, 16, Array, 'CREATE TABLE ci...') #6 /var/www/html/wp-content/plugins/civicrm/civicrm/packages/PEAR.php(223): PEAR->_raiseError(Object(DB_mysqli), NULL, -5, 16, in /var/www/html/wp-admin/admin-header.php on line 30
It seems like you have two problems.
As Pradeep mentioned in the comments - it seems like your underlying error is that the install is trying to create a database that already exists. I can't tell for sure, because the error in #4 is cut off.
Additionally, there's the second error (undefined function) which is unfortunately WordPress-specific and I don't know the solution. However, completely removing/creating the CiviCRM database again (assuming that it's separate from WordPress) is likely to solve the first problem, and the second problem won't occur.
Try adding following line in functions.php file of your child theme.
require_once(ABSPATH . 'wp-admin/includes/screen.php');
Which wordpress version and civicrm version are you trying?